1. Field of the Invention
The present invention relates to ultrasonic inspection equipment and method for performing ultrasonic inspection that is a kind of non-destructive examination, and more particularly to ultrasonic inspection equipment and method that use an array-probe ultrasonic sensor.
2. Description of the Related Art
In the field of ultrasonic inspection methods for inspecting various kinds of structural materials, an inspection method for imaging an internal state of a target to be inspected in a short period of time with high accuracy to inspect the target has been developed in recent years. Such an inspection method for imaging is typified by a phased array method, a synthetic aperture focusing technique, and the like (for example, refer to non-patent document 1).
The phased array method uses a so-called array-probe ultrasonic sensor having a plurality of piezoelectric vibration elements arrayed therein. The phased array method is based on the principles that wavefronts of ultrasonic waves individually transmitted from the piezoelectric vibration elements interfere with one another to form a composite wavefront, which then propagates. Therefore, by delaying the ultrasonic-wave transmission timing of each of the piezoelectric vibration elements so as to shift the timing of each ultrasonic wave, an incident angle of each ultrasonic wave can be controlled, thereby making it possible to focus the ultrasonic waves.
In addition, upon reception of ultrasonic waves, a reflection ultrasonic wave received by each of the piezoelectric vibration elements is shifted prior to its addition. Thus, as is the case with the transmission of ultrasonic waves, an incident angle of the received ultrasonic wave can be controlled. Also it is possible to receive the focused ultrasonic waves.
In general, the following methods are known as the phased array method: the linear scan method in which piezoelectric vibration elements of a one-dimensional array sensor are rectilinearly scanned; and the sector scan method in which transmission and receiving directions of an ultrasonic wave are changed in sector-like fashion. In addition, if a two-dimensional array sensor having piezoelectric vibration elements arrayed in a lattice-shaped pattern is used, ultrasonic waves can be three-dimensionally focused at any position, thereby providing a scan method that is suitable for a target to be inspected. Both of the above-described methods are capable of: scanning an ultrasonic wave at high speed without moving an array-probe ultrasonic sensor; and optionally controlling an incident angle of an ultrasonic wave and a position of the depth of focus without replacing an array-probe ultrasonic sensor. The linear scan method and the sector scan method are techniques that enable high-speed, high-accuracy inspection.
Next, the principle on which the synthetic aperture focusing technique is based is as follows. When an ultrasonic wave is transmitted in such a manner that wave motion of the ultrasonic wave widely diffuses into a target to be inspected, and a reflected ultrasonic wave signal of the ultrasonic wave is received, a position of a defect, which is a sound source of the received reflected ultrasonic wave, exists along a circular arc whose radius is the propagation distance of the reflected ultrasonic wave with a position of a piezoelectric vibration element which has transmitted and received the ultrasonic wave defined as the center of the circular arc. Based on the principle, an ultrasonic wave is transmitted and is received while the piezoelectric vibration element is successively moved, and each received waveform at respective positions of the piezoelectric vibration element is calculated by a computer so as to extend a waveform in the shape of a circular arc. As a result, intersection points of the circular arcs are concentrated on the position of a defect that is an ultrasonic-wave reflection source, thereby making it possible to identify the position of the defect. How the computer performs the calculation for the above process is described in the non-patent document 1.
With the above-described methods, each of which employs a sensor in which a plurality of piezoelectric vibration elements are arrayed, it is possible to three-dimensionally obtain a reflected ultrasonic wave signal indicative of a defect without movement of the sensor. However, in order to identify a three-dimensional reflection position from the reflected ultrasonic wave signal, for example, the following estimation is required: estimating the reflection position from a plurality of two-dimensional images of the reflection intensity distribution, positions of the two-dimensional images spatially differing from one another; or estimating the reflection position by converting the reflection intensity distribution into three-dimensional data, and then by three-dimensionally displaying the converted data.
For example, in the case of the linear scan and the sector scan in the phased array method, a plurality of two-dimensional reflection intensity images according to known scanning pitch can be acquired. Accordingly, a direction in which a reflected wave occurs can be identified by displaying images on a screen while the images are successively switched. However, there are limits to apply this method to some three-dimensional scanning other than the above-described scanning.
In such a case, reflected ultrasonic wave signals from a plurality of directions are subjected to interpolation processing or the like to create three-dimensional lattice-shaped data. The three-dimensional lattice-shaped data obtained is displayed as an image using a method such as volume rendering and surface rendering. There is also a method in which reflected ultrasonic wave signals are displayed as a three-dimensional point group without converting the reflected ultrasonic wave signals into lattice-shaped data. In any case, because the reflected ultrasonic wave signals are stored as three-dimensional ultrasonic inspection data, an inspector can check the three-dimensional ultrasonic inspection data from any direction after measurement (for example, refer to non-patent document 2).
However, it is difficult to judge only from the three-dimensional ultrasonic inspection data whether a peak of the reflection intensity distribution results from the reflection on an end face or a boundary surface of a target to be inspected or from the reflection on a defect. In particular, in the case of a complicatedly shaped target to be inspected, a large number of reflected ultrasonic wave signals (inner-wall echoes) resulting from such a shape are generated. Therefore, it is difficult even for an expert to discriminate between an inner-wall echo and a defect echo. However, software is developed that is capable of displaying three-dimensional shape data of a target to be inspected together with three-dimensional ultrasonic inspection data. Superimposing these two pieces of data on each other to make a comparison between them facilitates the discrimination between an inner-wall echo and an echo resulting from a defect (defect echo). Incidentally, data that has been separately created by general-purpose CAD (Computer Aided Design) is often read and used as three-dimensional shape data (for example, refer to the non-patent document 2).
Cited references are as follows:    Patent document 1:    JP-A-6-102258    Non-patent document 1:    “Norimasa KONDO, Yoshimasa OHASHI, Akiro SANEMORI, Digital Signal Processing Series, Vol. 12 ┌Digital Signal Processing in Measurement Sensor┘, PP. 143-186, May 20, 1993, Published by Shokodo    Non-patent document 2:    Potts, A.; McNab, A.; Reilly, D.; Toft, M., “Presentation and analysis enhancements of the NDT Workbench a software package for ultrasonic NDT data”, REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION; Volume 19, AIP Conference Proceedings, Volume 509, pp. 741-748 (2000)